Abstract: A lightweight crankless piston engine utilizes cam action to produce two piston strokes for one revolution of the engine output shaft, which shaft does not pierce the piston or the combustion chamber. Ease of assembly is enabled. One section of the cam is embodied in the cylinder liner and the opposing cam section is an extension of the output shaft housing. In its simplest mode, the engine is ported for fuel-air induction and exhaust. Cam induced piston rotation directly drives the output shaft while piston reciprocation is allowed without influencing such shaft.